1. Field of the Invention
The present invention relates an active material used for lithium ion batteries.
2. Description of the Related Art
Small secondary batteries are being used in increasing numbers to power portable equipment such as tools, toys, lighting, and more significantly in consumer electronic devices such as computer, camcorder, cellular phones and so on. More recently, secondary lithium batteries have received renewed interest with respect to employing a Li-insertion compound as anode instead of metallic Li to give the "lithium ion" batteries. This system, however, require careful selection of cathode and anode hosts. The layered LiMeO.sub.2 (Me: Co, Ni) [Mat. Res. Bull. 15 (1980) 783, J. Appl. Phys. 19 (1980) 305] and the three dimensional spinel oxide LiMn.sub.2 O.sub.4 [Mat. Res. Bull. 18 (1983) 461, Mat. Res. Bull., 19 (1984)179] having a mid discharge voltage at about 4V against lithium have become attractive cathode for lithium ion batteries. More recently, other type of cathode materials were investigated for a possible use in lithium ion system. These compounds are composed of Nasicon related 3D framework such as Li.sub.x M.sub.2 (PO.sub.4).sub.3 (M: Ti, V, Fe) and M.sub.2 (SO.sub.4).sub.3 (M: Ti, Fe) [Solid State ionic 92 (1996) 1]. In the undisclosed Japanese patents 9-134 725, 9-134 724, 9-171 827 and 9-134724 LiMPO.sub.4, M: Ni, Co, Mn, Cu were claimed. These materials have an Olivine structure totally different from materials of the present invention.
As for primary lithium batteries, there were candidates of cathode materials of phosphate or arsenates of heavy metals such as Cu.sub.3 (PO.sub.4).sub.2, Ag.sub.3 PO.sub.4, Cu.sub.3 (AsO.sub.4).sub.2 and Ag.sub.3 AsO.sub.4 [U.S. Pat. No. 3,736,184]. However these materials have not been put into practice so far.